Automotive coatings generally comprise an electrodeposition coating (EDC) primer, a primer surfacer layer (filler coating layer) applied thereto and a top coat applied thereto comprising a wet-on-wet applied color- and/or special effect-imparting base coat layer and a protective, gloss-imparting clear coat layer. During automotive body OEM coating (automotive body original coating; OEM=original equipment manufacture), such multi-layer coatings are conventionally very largely automatically applied in industrial coating facilities, wherein at least on application of the base coat a number of differently colored base coats corresponding to the color shade program is processed. In some coating facilities, even the primer surfacer is here applied using two or more primer surfacers in each case of a different color shade. One or more of the base coats used is/are here assigned to each individual primer surfacer while taking account of the color shades in question; i.e., the attempt is made to ensure the greatest possible similarity in color shade between the color shade of the primer surfacer and that of the base coat. The aim here is to ensure that any stone chip damage accompanied by detachment of the base coat which occurs during use of the motor vehicle remains as visually unobtrusive as possible.
Automotive plastics coatings generally comprise an electrically conductive plastics primer and a top coat applied thereto comprising a wet-on-wet applied color- and/or special effect-imparting base coat layer and a protective, gloss-imparting clear coat layer, although, dependent on the plastics type and its pretreatment, it may also be possible to omit the plastics primer. As described in the preceding paragraph for the automotive body OEM coating, the original coating of automotive plastics parts is conventionally performed very largely automatically in industrial coating facilities, wherein in this case too at least on application of the base coat a number of differently colored base coats corresponding to the color shade program is processed. Here, in some coating facilities two or more plastics primers each of a different color shade are used. Similarly, two or more plastics parts for primerless coating (plastics parts to be coated primerless; plastics parts not to be coated with primer) and which each differ in plastics material color may be used. As described in connection with primer surfacer coating, the base coats are also assigned on the basis of color shade in the case of differently colored plastics primers or differently colored plastics parts for primerless coating.
There are, however, also industrial coating facilities in which the effort of tailoring the colors of primer surfacer and base coat or of plastics primer or plastics parts for primerless coating and base coats to one another is avoided. For example, for reasons of rationalization, just one primer surfacer or just one plastics primer or plastics parts for primerless coating of only one color are used here. The primer surfacer, plastics primer or plastics parts for primerless coating are here generally of a color shade which is a compromise (compromise color shade). Such a compromise color shade is usually a shade of grey. In the case of base coats with elevated hiding power, this causes no problems with regard to color shade feasibility (achievability of the nominal color shade). However, depending on the compromise color shade and the base coat color shade, color shade feasibility problems may occur in the case of base coats with low hiding power. For example, such a base coat with a problematic color shade must be applied in a very high film thickness to achieve color shade consistency. Color shade consistency or achieving color shade consistency here means that a visually imperceptible color shade deviation ΔE from the color shade of a coating layer applied from the base coat in question in a black/white opaque or even greater film thickness is ensured. In the case of solid color shades, ΔE values of <0.4 determined at an illumination angle of 45° to the perpendicular and an observation angle of 45° relative to the specular reflection are sufficiently small and thus represent color shade consistency in the above sense. In the case of special effect color shades (color shades dependent on observation angle; pigment content comprises at least one special effect-imparting pigment) ΔE values are sufficiently small if they, when determined at an illumination angle of 45° to the perpendicular and at observation angles of 15, 25, 45, 75 and 110° relative to the specular reflection, are in each case <1.
The elevated base coat film thickness required to achieve color shade consistency not only entails high base coat consumption which is undesirable from both an environmental and an economic standpoint, but may also entail disadvantages for the technological properties of the multi-layer coating or may simply mean non-compliance with the specified nominal base coat film thickness.
By the way, it is not possible to achieve any base coat film thickness in automatically-operated industrial coating lines. The latter are operated in work cycles, i.e. only a limited period of time is available for the coating of each single substrate. The work cycle time, number and type of spray equipment and the area to be coated determine the maximum base coat film thickness that can be achieved with a given water-borne base coat. In state-of-the-art automatically-operated industrial coating lines the maximum film thickness that can be achieved with conventional water-borne base coats is in the range of 20 to 30 μm. Higher base coat film thicknesses can only be achieved by increasing the number of work cycles per substrate which means loss in productivity.
One theoretical approach would also be to adjust the pigment/binder ratio in the base coat in favor of the pigment content and in this way to counteract an elevated base coat film thickness. From a practical standpoint, however, this is not a suitable solution, because multi-layer coatings produced with such highly pigmented base coats exhibit technological weaknesses, in particular with regard to adhesion and stone chip resistance. Moreover, optical appearance is impaired, this being visually perceptible as texture and matting and instrumentally detectable as lower distinctness of image (DOI).
WO 2008/051346 discloses a method for producing primer surfacer-free multi-layer coatings consisting of an electrodeposition coating layer, a first water-borne base coat layer, a second water-borne base coat layer and a clear coat layer on metal substrates. The second water-borne base coat layer is here applied in a comparatively low film thickness from an unmodified water-borne base coat with low hiding power, while the first water-borne base coat layer is applied from a modified water-borne base coat. The modified water-borne base coat is a mixture of the unmodified water-borne base coat, a further water-borne base coat which is likewise unmodified but has sufficient hiding power and a pigment-free admixture component providing polyisocyanate or binder.